In the field of lighting apparatus, a study has been conducted for using LED bare chips for a lighting apparatus by densely mounting many one-side-electrode type LED bare chips on a printed wiring board (hereinafter simply called “a wiring board”) by a flip chip method using ultrasonic bonding. In the lighting apparatus, each LED chip has several hundred μm square in size and has p-electrodes and n-electrodes on one surface.
FIG. 18A is an enlarged plan view showing an LED chip 710 flip-chip mounted on a wiring pattern 701 for a p-electrode and a wiring pattern 702 for an n-electrode, both attached to a wiring board 700. In FIG. 18A, to make a clear distinction between the LED chip 710 and the wiring patterns 701 and 702, the LED chip 710 is drawn with thick lines and the p-electrode 711 and the n-electrode 712, which are disposed on the reverse side (the under surface) of the LED chip 710, are drawn as viewed through the LED chip 710.
As shown in FIG. 18A, the p-electrode 711 and the n-electrode 712 on the one surface of the LED chip 710 are disposed so as to oppose each other and have a distance d therebetween (e.g. d is approximately 20 μm, where the LED 710 is 300 μm square).
Meanwhile, the wiring patterns 701 and 702 are disposed so as to oppose each other and have the distance d therebetween in accordance with the shapes of the corresponding p-electrode 711 and n-electrode 712. Here, the p-electrode 711 and the n-electrode 712 on the LED chip 710 are electrically connected to the wiring pattern 701 and the wiring pattern 702 respectively so as to have surface contact, using the ultrasonic bonding.
Such a lighting apparatus is generally supplied with large amount of electricity to gain high optical output. Therefore, large amount of heat is liberated, and this might cause cracks at the junction of the LED chip 710 and the wiring board 700 due to the difference of thermal expansions. To avoid this problem, conventional arts increase the power level of the ultrasonic bonding, thereby increase the bonding strength.
However, if the bonding strength is increased by increasing the power level of the ultrasonic bonding, the LED chip 710 might be rotated a certain angle, as shown in FIG. 18B, from the position that the LED chip 710 should take when it is mounted (“normal mounting position”, the position illustrated in FIG. 18A), and the p-electrode 711 and the n-electrode 712 might short out (in areas indicated by a sign a). The reason why this happens is the following. A flip chip bonder is used for mounting the LED chip 710. The flip chip bonder bonds the LED chip 710, which is attracted to the tip of a collet, to the wiring board 700 by applying ultrasonic vibration while placing and pressing the LED chip 710 onto the designed mounting location. If the power level is increased, the LED chip 710 wobbles by rotating a little around the collet as the rotation axis, and the LED chip 710 might be mounted after it is rotated from its normal mounting position. If the short circuit is caused, the LED does not function as a matter of course, and processing such as remounting will be required. This considerably reduces yields.
Such a problem can be caused not only in the case of mounting the LED chips, but also in the case of mounting optical semiconductors such as semiconductor lasers by the above-described method.